Optimization of biochar and fly ash to improve mechanical properties and CO2 sequestration in cement mortar

The novelty of this study is to find out the optimum amount of biochar and high calcium fly ash to improve mechanical performance and promote CO2 uptake in cement mortar. The potential of biochar-fly ash mixes was optimized using response surface methodology (RSM) considering the influence of different amounts of fly ash (FA), biochar (B), and their interactions to achieve the optimal mix design with improved performance and carbonate mineralization. Additionally, the effect on the mineralization of different polymorphs of calcium carbonate was also investigated. The experimental results indicate that the accelerated carbonation of high calcium fly ash mortar enriched with biochar enables the effective utilization of biochar up to 5% for enhancing both the mechanical properties and CO2 sequestration. However, under normal curing conditions, it was observed that the optimal amount of biochar that can be used is <1%. This is a promising approach in terms of utilizing waste material and reducing cement content in the mix design together with the additional benefit of storing 50% of ambient CO2 permanently in the cementitious matrix. Carbonation of biochar-fly ash cement composite substantially improves fracture strength, and modulus of elasticity compared to uncarbonated cement mortar at 7 and 28 days. The present study demonstrates the advantages of synergistically combining biochar and fly ash in the development of low-carbon, sustainable cementitious building materials that have the potential to transform building structures into carbon sinks in the future.